For many decades, a convenient, efficient and cost-effective means of producing shot from various metals has been sought. With the discovery that lead possesses toxic properties, the search has intensified considerably, particularly over the past three years. The provision of a cost-effective and highly efficient means for producing metal shot basically involves separating a stream of molten metal into a substantial quantity of discrete droplets per unit time, and gradually extracting the heat from those droplets. This must be done while shaping the droplets into virtually perfectly spherical bodies until they freeze, and preventing both inter-particle collisions during the freezing process and oxidation of the metal both during and after hardening. There are three basic problems to solve in doing this: (1) the molten stream of metal poured from the furnace or crucible must be separated into individual droplets of a pre-determined diameter, from which shot may be formed; (2) the heat must be extracted from the droplets of molten metal gradually and in a highly controlled manner, and (3) the latter must be done in a manner which sufficiently separates the droplets from each other so as to prevent deformations resulting from inter-particle collisions.
Existing devices have failed to find satisfactory solutions to these three basic problems. At present research laboratories are pouring thin streams of metal through heat-exchange fluids in an effort to cool the metal sufficiently to form solid shot of uniform diameter and exacting tolerances. However, with no control over the free fall of the metal through the heat exchange fluid, and lacking means to resolve the thin stream of metal into separate droplets, many of the particles in such an apparatus either fuse together prior to freezing, collide with each other in the process of freezing, or fail to freeze homogeneously. Moreover, there is little control over the diameter of the shot produced, or over the uniformity of cooling, resulting in shot of varying diameters and poor sphericity.
Several different types of shot-producing devices have been reported, such as the shot tower. It is exceedingly cumbersome, being 125-150 feet high, and requires the time, expense and inconvenience of hoisting and reheating the metal used and preparing and maintaining special housing for the apparatus. Moreover, it is limited by its inability to produce shot from any metal but lead, and is unable to generate large shot to exacting tolerances.
Alternatively, other machines, more compact than the shot tower, are relatively inefficient in production capacity over time and in percentage of shot meeting specified tolerance requirements. These devices are compact and capable of making shot from a variety of metals. However, they too are characterized by an inefficiency in production capacity and percentage of shot produced meeting specified tolerances and, in addition, are extremely complex, prone to malfunction, and are extraordinarily expensive. Moreover, virtually all of the machines listed above coat the shot metal with graphite with its attendant disadvantages of handling, and cost. Casting and oil-bath techniques are also notably lacking in efficiency and productivity, being slow and failing to yield a high percentage of large shot meeting specified tolerances. The present invention provides methods and apparatus for producing shot which overcome the above-noted deficiencies in the art.